In a geographically divided communication system, e.g., a cellular wireless system, a frequency reuse scheme may be implemented by different communication areas according to the geographical division to improve the capacity of the wireless communication system. Each communication area is called a cell. When the frequency resources are allocated using a reuse factor of 1, different cells use the same frequency. At this time, signal interference between different cells may be caused.
Due to the feature of being able to overcome multi-path propagation and simplicity in equalization, the OFDM technology is drawing more and more attention. The OFDM divides the time-frequency resources of a wireless communication system into several orthogonal narrow-band sub-channels. High-rate data flows are transmitted in parallel in each sub-channel after a serial-to-parallel conversion. Since the narrow-band feature of the sub-channel can conquer the multipath affects and ensure the orthogonality between sub-channels, thereby ensuring that there is little interference between intra-cell users. The international patent application PCT/CN2004/000128, titled “Multiplexing scheme in a communication system”, describes a method for allocating time-frequency resources in an OFDM communication system. The method ensures little intra-cell interference while randomizing inter-cell interference. A feature of the method is that no frequency planning is needed, which makes the method especially applicable to the case with a frequency reuse factor of 1. Specifically, the method includes the steps of: generating a generic time-frequency pattern; generating a set of orthogonal time-frequency patterns from the generic time-frequency pattern; performing a random cyclic shift of the set of orthogonal time-frequency patterns in each Transmission Time Interval (TTI); and allocating the obtained orthogonal time-frequency patterns to at least one user and/or traffic channel.
In the PCT application, the generic time-frequency pattern is generated by a Costas sequence. The random variable cyclic shift of the orthogonal time-frequency patterns may be performed in the time domain or in the frequency domain. The obtained orthogonal time-frequency patterns may be allocated to at least one user and/or traffic channel in a random manner.
One embodiment of the above PCT application will be described hereinafter.
The time-frequency resources of an OFDM wireless communication system can be seen as a two-dimensional time-frequency plane. In the solution, the bandwidth allocated to the whole wireless communication system is pre-divided into N sub-carriers in frequency domain, and n consecutive sub-carriers construct a sub-band. The whole available frequency resources of the user and/or traffic channel may be divided into [N/n]=F sub-bands. Each sub-band may be considered as a basic frequency unit. Meanwhile, a TTI includes M basic time units and each basic time unit may be an OFDM symbol time. Thus the time-frequency plane in a TTI is a set of two-dimensional grids in M basic time units and F basic frequency units.
A time-frequency pattern is defined as a set of two-dimensional grids in a time-frequency plane. The time-frequency resources in a TTI may be divided into a set of time-frequency patterns orthogonal to each other. Thus the time-frequency resources may be shared by allocating the time-frequency patterns to at least one user and/or traffic channel.
In the PCT application, each time-frequency pattern may be expressed as a sequence of indices of basic frequency units used in each basic time unit according to the order of the basic time unit. For example, the time-frequency pattern corresponding to the sequence P={p(0),p(1),p(2), . . . ,p(M−1)} is the index p(k) of the occupied frequency unit in the kth basic time unit.
The time-frequency pattern may be a Costas sequence with length of F, and the Costas sequence may be expressed asTFPgeneric={p(0),p(1),p(2), . . . ,p(F−1)},
in which the Costas sequence {pi} with length of F is defined as a permutation sequence of integers {0,1,2, . . . F−1} and when i≠j and i+n,i,j+n,j∈{0,1, . . . ,F−1}, it satisfies that pi+n−pi≠pj+n−pj.
The time-frequency pattern TF00 is generated by the TFPgeneric. The length of the TF00 is M. When M<=F,TF00(k)=p(k), where k=0,1,2, . . . (M−1). That is, the time-frequency pattern is generated by the sequence segment including the first M elements of the TFPgeneric. When M>F, TF00(k)=p(k) if k=0,1,2, . . . (F−1), while TF00(k)=p(M−k−1) if k=F,(F+1), . . . ,(M−1). That is, the time-frequency pattern is generated by TFPgeneric and a converse sequence of its first (M−F) elements.
Writing TF00(k)=s00(k), where k=0,1,2, . . . ,(M−1), the time-frequency pattern generated by f cyclic shifts in the frequency domain of the time-frequency pattern TF00 may be written as TFf0(k)=sf0(k) k=0,1,2, . . . (M−1), where sf0(k)=(s00(k)+f)mod F.
As can be seen from the above, different frequency-domain cyclic shifts generate different orthogonal time-frequency patterns, and totally F orthogonal time-frequency patterns may be generated. The set of orthogonal time-frequency patterns may be allocated to at least one user and/or traffic channel in one cell. In each TTI, available time-frequency patterns in one cell may be generated by random cyclic shifts in the time domain of the set of orthogonal time-frequency patterns. For example, the pattern generated by t cyclic shifts in the time domain of the time-frequency pattern TF00 may be written asTFft(k)=sft(k) k=0,1,2, . . . (M−1)
where sft(k)=sf0((k+t)modM), and the time-frequency pattern TF00 may be performed M different cyclic shifts in the time domain.
In order to reduce collisions of the time-frequency patterns between cells, each cell has a special multi-level pseudo-random sequence for controlling the shifts t in the time domain which changes as the TTI changes. The multi-level pseudo-random sequence has pseudo-random property and thus even though two cells select the same cyclic shift value in the time domain in a certain TTI and cause synchronization, they have little possibility of synchronization in next TTI.
Although the probability of selecting the same cyclic shift value in the time domain in a certain TTI by two cells is 1/M, once the case occurs, the time-frequency patterns of the two cells are the same. Such case leads to complete interference of the traffic channels of the two cells, and thereby leading to higher error rate. In order to further decrease the interference of the traffic channels, a traffic channel may randomly select one of the generated F orthogonal time-frequency patterns in each TTI. As such, in the case that the time-frequency resources of the cell are not fully used, the probability of full overlap between a time-frequency pattern and that of an adjacent cell is smaller than 1/M, but larger than 1/(M*F). For example, when M=12, F=15, the number of the total available time-frequency patterns is only 15*12, and even in a most random selection case, the probability when the time-frequency pattern used by a traffic channel in a certain cell is also used by a traffic channel in the adjacent cell is larger than 1/(15*12).
As can be seen from the above description, the time-frequency resources may be allocated with no resource planning in accordance with the above PCT application. The method in the above PCT application utilizes the random time shift to avoid the complete overlap of time-frequency patterns between cells and utilizes the random frequency shift to further reduce the probability of the overlap between traffic channels.